The present invention relates to a motor control apparatus for controlling brushless motors incorporated in air conditioners, refrigerators, washing machines, blowers, etc. by using an inverter circuit.
FIG. 33 is a block diagram showing the configuration of a conventional motor control apparatus for driving a brushless motor. In the following descriptions, the conventional motor control apparatus shown in FIG. 33 is referred to as a first conventional technology. In FIG. 33, numeral 101 designates an AC power source, numeral 102 designates an inductor, numeral 103 designates a rectifying diode, numeral 104 designates a smoothing capacitor, numeral 106 designates an inverter circuit, numeral 107 designates a brushless motor, and numeral 108 designates a position sensor. In the case when the AC voltage supplied from the AC power source 101 is converted into a DC voltage by using the rectifying diode 103 and the smoothing capacitor 104 so that a DC power is input to the inverter circuit 106, the current supplied from the AC power source 101 flows only when the voltage of the smoothing capacitor 104 is lower than the supplied AC voltage. Hence, the current supplied from the AC power source 101 has harmonic components. Hence, in the first conventional technology, the inductor 102 is provided between the AC power source 101 and the rectifying diode 103 to reduce the harmonic components and to improve power factor. As described above, in addition to the rectifying diode 103, the inductor 102 and the smoothing capacitor 104 are used in the rectifying circuit 105 of the first conventional technology. Furthermore, in the case when the brushless motor 107 is driven by the inverter, the rotation angle information of the rotor is necessary. For this purpose, in the first conventional technology, the rotation angle was detected by using the position sensor 108. As an example of this first conventional technology, a motor control apparatus disclosed in Japanese Laid-open Patent Application No. Hei 9-74790 is proposed.
The inductor 102 and the smoothing capacitor 104 of the rectifying circuit 105 for use in the first conventional technology are large components having a large inductance and a large capacitance, respectively, in many cases. As a result, the conventional motor control apparatus is frequently apt to be large in size and high in price. In the field of motor control apparatuses, a rectifying circuit incorporating compact components like an inductor having a small inductance and a capacitor having a small capacitance or a rectifying circuit not incorporating these components has been demanded from the viewpoint of making the apparatus smaller in size and lower in cost.
In this situation, such a motor control apparatus incorporating neither inductor nor smoothing capacitor as shown in FIG. 34 is proposed as a second conventional technology. As an example of this second conventional technology, a motor control apparatus disclosed in Japanese Laid-open Patent Application No. Hei 10-150795 is proposed. Since no smoothing capacitor is used in the second conventional technology, the input voltage supplied to the inverter circuit 106 is not a DC voltage but a pulsating voltage. If this kind of pulsating voltage is input to the inverter circuit 106, when the input voltage supplied to the inverter circuit 106 is low, the inverter circuit 106 cannot generate a desired voltage to be applied to the brushless motor 107 in some cases. In the case when the desired voltage cannot be obtained by the second conventional technology, the phase of the voltage to be applied to the brushless motor 107 is advanced. By advancing the phase of the voltage to be applied to the brushless motor 107, the so-called weak field state can be obtained, whereby the voltage required to be applied to the brushless motor 107 can be made lower. Hence, with the second conventional technology, the brushless motor 107 can be driven continuously even when the input voltage supplied to the inverter circuit 106 is low. However, in the second conventional technology, in the case when the input voltage supplied to the inverter circuit 106 becomes a predetermined value or less, the switching operation of the inverter circuit 106 is stopped. This is because motor drive in the weak field state is limited. As described above, the second conventional technology is configured so that no voltage is applied to the brushless motor 107 in the case when the input voltage supplied to the inverter circuit 106 becomes the predetermined value or less.
In addition, a motor control apparatus not using a position sensor is demanded from the viewpoint of making wireless and making the cost lower. In this situation, a method of estimating the rotor position of a brushless motor by detecting the motor current is proposed as a third conventional technology. In the third conventional technology, the rotor position of the motor is estimated by using a calculation equation for estimating the phase derived on the basis of a voltage equation from a motor current, a voltage applied to the brushless motor at the time when the motor current flows, and motor constants, such as the resistance, inductance, etc. of the brushless motor. An example of this third conventional technology is disclosed in a thesis “Control of a sensorless salient-pole brushless DC motor on the basis of estimation of speed electromotive force (Back EMF Estimation-Based Sensorless Salient-Pole Brushless DC Motor Drives)” by Takeshita, Ichikawa, Lee and Matsui, Thesis Journal, Vol. 117-D, No.1, pages 98 to 104, issued by the Institute of Electrical Engineers of Japan in 1997 (T.IEE Japan, Vol.117-D, No.1, '97).
As described above, in the first conventional technology, the rotor position of a brushless motor is detected by using a position sensor, and an inductor and a smoothing capacitor are used to convert the input voltage supplied to an inverter circuit into a DC voltage. Hence, since the inductor and the smoothing capacitor are large components having a large inductance and a large capacitance, respectively, it is difficult to make the motor control apparatus incorporating these components smaller.
In addition, the second conventional technology is a motor control apparatus in which the rotor position of a brushless motor is detected by using a position sensor, without using large components such as an inductor and a smoothing capacitor. This technology is thus effective from the viewpoint of making the apparatus smaller in size and lower in cost. However, since the input voltage supplied to the inverter circuit pulsates in the second conventional technology, this causes a problem of stopping voltage application to the brushless motor when the input voltage is a predetermined value or less.
A problem described below occurs in the case when a sensorless motor control apparatus is tried to be built so as to be made smaller in size and lower in cost by combining the second conventional technology configured not to use any inductor and smoothing capacitor with the third conventional technology configured to carry out sensorless motor drive. In the motor control apparatus having this kind of configuration, the rotor position cannot be estimated in periods during which voltage application to the brushless motor is stopped. Hence, sensorless drive for the brushless motor was impossible. In other words, in the case when the input voltage supplied to the inverter circuit pulsates, a sensorless motor control apparatus cannot be built by the simple combination of the second conventional technology and the third conventional technology.